1. Field of the Invention
The present invention relates to a tracking control apparatus of a rotary head type recording unit using pilot signals for detecting a tracking error.
2. Description of the Prior Art
A VTR with a pair of rotary heads for forming slant tracks on a magnetic tape wound around a tape guide drum is known. In addition, a digital VTR which records a digital video signal, a digital audio signal, and control/display digital sub data in tracks on a time division basis has been proposed. Moreover, for a digital VTR, a tracking control apparatus which records a tracking control pilot signal in respective regions,disposed at opposite ends of a track (these regions are referred to as ATF areas) is also known.
FIG. 16 is a schematic diagram showing the results of a conventional recording system for recording an ATF pilot signal on each track. In FIG. 16 and the description that follows, x represents the scanning direction of a pair of rotary heads; y represents the traveling direction of a magnetic tap, A and B represent tracks formed by the rotary heads; Dx represents the offset distance between the ends of a track A and a track B just adjacent thereto; a hatched region represents a record area of a pilot signal. The extended direction of a gap formed on one of the heads in a pair differs from that of the other head in that pair so as to provide an azimuth loss which reduces crosstalk between two tracks. In FIG. 16, for simplicity, slant tracks are horizontally depicted.
The length of the record region of a pilot signal is defined as multiples of the offset distance Dx. The offset distance Dx is mechanically defined by the rotation of the drum and the travel of the tape of a VTR. As opposed to the present invention, in the prior art shown in FIGS. 16 and 17, the pilot signal has only one frequency. In this example, the frequency f is set to approximately 1 MHz. On each track A, the pilot signal is recorded in the first DX region and the fourth DX region. 0n each track B, the pilot signal is recorded in the third DX region. The pilot signal is repeatedly recorded in such a sequence. The length necessary for controlling a tracking operation is 5 Dx. This length is referred to as an ATF area. FIG. 16 shows a record pattern of the pilot signal on the side of the track where the rotary head enters (namely, the head entering side). However, the record patter on the head leaving side is the same as that on the head entering side.
When recording signals are reproduced from a magnetic tape on which the pilot signal is recorded according to the record pattern as shown in FIG. 16, a tracking error is detected as follows. When one head with the same azimuth angle as a track A scans this track, it reproduces a pilot signal from this track, a crosstalk of a pilot signal from one of the two adjacent racks, and then a crosstalk of a pilot signal from the other. When the other head with the same azimuth as track B scans this track, it reproduces a pilot signal from this track, a cross talk of a pilot signal from one of the two adjacent racks, and then a crosstalk of a pilot signal from the other.
In the ATF area, the first reproduction output of the pilot signal on the same azimuth track is relatively large. This reproduction output is used as a timing reference. With this timing reference, the levels of the crosstalks of the two adjacent tracks are sampled and held. By using the difference of the outputs which were sampled and held, the direction and amount of a tracking error can be detected.
As shown in FIG. 16, when the record region of a pilot signal is set to the offset distance Dx, unless the value of Dx is satisfactorily large, due to deviation of rotation of the rotating drum, deviation of record pattern, and so forth, the accuracy of the timing reference used for the sampling/holding process and the accuracy of the sampling/holding process itself will degrade. As a result, a tracking error cannot be correctly detected.
As a method for solving this problem, the length of the record region of a pilot signal can be doubled (namely, 2 Dx) as shown in FIG. 17. However, the length of the ATF area likewise becomes long. To maximize the record area for digital information signals on each track, the length of the ATF area should be as short as possible.
In addition, when a pilot signal has been recorded on a magnetic tape as shown in FIGS. 16 and 17, if the head positions deviate from the respective track positions, it takes a long time until a proper tracking occurs. In other words, when one head with an azimuth angle different from a particular track enters this track, the reproduction output of the pilot signal as a timing reference is small due to azimuth loss and thereby the sampling pulse for detecting a crosstalk level cannot be formed. Thereafter, due to a speed error of the capstan, the tracking state gets gradually disordered. After the pilot signal for a timing reference is reproduced, the tracking control operation can be performed. Thus, in such situations as just after the capstan motor starts rotating and when a large track deviation as large as one track takes place due to an external disturbance, it takes a long time until a normal tracking state occurs.